Ah - you know I'd not really covered this at all. I hadn't really thought that you'd want to get about the table prone but I guess it's an option
I'd probably allow Prone Characters to crawl about at half speed. So a single Action for a 4 Speed Character would allow 2" of movement. The rule for dice rolls suggests fractions round down but in the case of movement you can take Fractional moves so a 3 Speed Character could crawl 1 1/2" as a single action.
I'll update the FAQ and probably the Electronic rules to reflect this.
Good pick-up!